From the idea of carbon neutrality, strong attention has been given to polylactic acids (PLAs) which utilize a renewable resource, biomass, as starting material. PLAs are relatively low cost and heat resistant having a melting point of about 170° C.; therefore, they are expected as a biodegradable polymer which can be melt-molded. In addition, it is known that a stereocomplex polylactic acid can be obtained by mixing poly-L-lactic acid and poly-D-lactic acid (JP S61-36321A, JP S63-241024A and JP 2000-17163A). Stereocomplex polylactic acid are known to exhibit a higher melting point and higher crystallizability as compared to single polymers and provide useful molded articles as fibers, films and resin molded articles. For both of the starting materials thereof, L-lactic acid and D-lactic acid, there is a demand for a method of producing them with high purity and high efficiency.
In nature, there exist bacteria which efficiently produce lactic acids, such as lactic acid bacteria, and some of those lactic acid production methods utilizing such bacteria have already been put to practical use. Examples of bacteria which efficiently produce L-lactic acid include Lactobacillus delbrueckii. In addition, as bacteria which efficiently produce D-lactic acid, microbes such as Sporolactobacillus laevolacticus are known (WO 2007/043253). In any of these cases, the amount of the accumulated lactic acid reached a high level in anaerobic culture; however, since D-lactic acid and L-lactic acid are produced as by-product in L-lactic acid fermentation and D-lactic acid fermentation, respectively, the optical purity is lowered. In addition, it is extremely difficult to separate these lactic acids.
Therefore, as a production method of lactic acid having a high optical purity, it has been examined to introduce a gene encoding L-lactate dehydrogenase or D-lactate dehydrogenase into a yeast which does not intrinsically have lactic acid-producing ability and subject the yeast to L-lactic acid and D-lactic acid fermentation (WO 2007/043253, JP 2007-074939A, WO 2004/104202 and Ishida N, et al., Journal of Bioscience and Bioengineering (2006), 101, 172-7). With regard to L-lactic acid fermentation by genetically engineered yeast, by introducing a highly active gene originated from Xenopus laevis which encodes the L-lactate dehydrogenase, lactic acid fermentation can be efficiently performed with high optical purity (WO 2007/043253). On the other hand, with regard to D-lactic acid fermentation by genetically engineered yeast, although D-lactic acid having high optical purity can be obtained in the same manner as in the case of L-lactic acid fermentation, the yield thereof is a problem (JP 2007-074939A, WO 2004/104202 and Ishida N, et al., Journal of Bioscience and Bioengineering (2006), 101, 172-7).
It could therefore be helpful to provide highly productive D-lactic acid fermentation using a polypeptide having a D-lactate dehydrogenase activity higher than that of conventional polypeptides and a polynucleotide encoding the polypeptide.